1. Field of the Invention
The present invention relates generally to threaded API casing connections, but more specifically to the couplings for API Buttress threaded casing to be used as a combination drill string and casing string—i.e., Drilling With Casing (DWC).
2. Description of the Related Art
Presently the conventional method for drilling oil and gas wells is to use drill pipe specifically designed for and dedicated to drilling the well bore. Upon drilling a well to completion, the drill pipe is pulled from the well and transferred to the next location for drilling another well. The drill pipe is thus used until it is worn out. The open hole left by the drill pipe is sealed off by running a string of casing pipes to the bottom of the hole and cementing the casing string in place.
In contrast to the above procedure, it is the purpose of new technology to eliminate the use of the above-described drill pipe and instead use the casing string for both drilling the well and casing off the open hole. The procedure is commonly referred to as “Drilling With Casing” (DWC). This procedure has been tried in various parts of the world but with limited success. However DWC offers so much potential for reducing drilling costs that interest remains high throughout the industry and many new projects are aimed at advancing the technology.
In conventional casing usage, the casing and its connections are subjected only to static loads comprising tension, torsion, compression, bending, pressure and any combination thereof. In DWC usage, the casing and connections are not only subject to all of the above static loads, but also to dynamic loads due to rotating the casing at 100 to 150 RPM while drilling the well bore. As the casing rotates and advances down the well bore, the casing string and particularly the connections, which have a larger outside diameter than the casing, are subject to cyclic fatigue loads, severe abrasion wear and impact loading, then finally to all the static loads mentioned above after the casing is set and cemented in the well.
This invention is directed at one of the primary problems associated with DWC—the connections which join each length of casing, one to another. Experience to date with DWC has demonstrated a need for a more robust, yet economical casing connection to withstand the additional rigors of dynamic loading and frictional wear caused by rotating the string while drilling.